(1) Field of the Invention
This invention relates to a process for forming fibers, and fibers formed by the process. More particularly, this invention relates to such a process in which said fiber is formed by spinning a melt or solution of a polymer through a capillary spinneret having a length/diameter (L/D) ratio equal to or greater than about 60:1, and fibers formed by such method
(2) Prior Art
Melt and solution methods of spinning fibers are known. For example, PAN has been spun conventionally using either wet spinning (e.g., 9.5% PAN in sodium thiocyanate-water (50:50) spun into 10% sodium thiocyanate in water at -2.degree. C. for coagulation) or dry spinning (e.g., 30% PAN in diethylformamide spun at 103.degree. C.) Typical properties of the resultant fibers are 2.4-3.7 g/denier tenacity and 42-53 g/denier tensile molecules. See Table 1 on page 155 of S.S. Chari et al., Fibre Science and Technology, Vol. 15, pp. 153-60 (1981). Mention is also made of PAN fibers in Smith et al. U.S. Pat. No. 4,344,908 (1982) concerned primarily with polyethylene fibers. Also concerned primarily with polyethylene fibers is U.S. Pat. No. 4,413,110 of Kavesh and Prevorsek (Nov. 1, 1983).
Zwick et al. in Soc. Chem. Ind., London. Monograph No. 30, pp. 188-207 (1968) describe the spinning of polyvinyl alcohol by a Phase Separation technique said to differ from earlier Wet Spinning, Dry Spinning and Gel Spinning techniques. The reference indicates that the earlier systems employ 10-20%, 25-40% and 45-55% polymer concentrations, respectively, and that they differ in the manner in which low molecular weight materials (solvents such as water) are removed. The reference also indicates some earlier systems to be restricted in spinneret hole size, attenuation permitted or required, maximum production speed and attainable fiber properties.
The Phase Separation process described in Zwick et al. (see also UK Patent Specification No. 1,100497) employs a polymer content of 10-25% (broadly 5-25% in the Patent which covers other polymers as well) dissolved at high temperatures in a one or two-component solvent (low molecular weight component) system that phase separates on cooling. This phase separation took the for of polymer gellation and solidification of the solvent (or one of its components), although the latter is indicated in the patent to be optional. The solution was extruded through apertures at the high temperature through unheated air and wound up at high speeds hundreds or thousands of times greater than the linear velocity of the polymer solution through the aperture. Thereafter the fibers were extracted to remove the occluded or exterior solvent phase, dried and stretched. An earlier, more general description of Phase Separation Spinning is contained in Zwick, Applied Polymer Symposia, no. 6, pp. 109-49 (1967).
Modifications in the spinning of hot solutions of ultrahigh molecular weight polyethylene (see Examples 21-23 of UK No. 1,100,497) have been reported by Smith and Lemstra and by Pennings and coworkers in various articles and patents including German Offen No. 3004699 (Aug. 21, 1980); UK Application 2,051,667 (Jan. 21, 1981); Polymer Bulletin, vol. 1, pp. 879-880 (1979) and vol. 2, pp. 775-83 (1980); and Polymer, Vol. 21, pp. 3-4 (1980). Copending commonly assigned applications of Kavesh et al., U.S. Pat. Nos. 4,413,110 and 4,551,296 describe processes including the extrusion of dilute, hot solutions of ultrahigh molecular weight polyethylene or polypropylene in a nonvolatile solvent followed by cooling, extraction, drying and stretching. While certain other polymers are indicated in U.S. Pat. No. 4,413,110 as being useful in addition to polyethylene or polypropylene, such polymers do not include polyvinyl alcohol or similar materials
While U.K. Patent No. 1,100,497 indicates molecular weight to be a factor in selecting best polymer concentration (page 3, lines 16-26), no indication is given that higher molecular weights give improved fibers for polyvinyl alcohol. The Zwick article in Applied Polymer Symposia suggests 20-25% polymer concentration as optimum for fiber-grade polyvinyl alcohol, but 3% polymer concentration to be optional for polyethylene. The Zwick et al article states the polyvinyl alcohol content of 10-25% in the polymer solution to be optimal, at least in the system explored in most detail where the solvent or a component of the solvent solidified on cooling to concentrate the polyvinyl alcohol in the liquied phase on cooling before the polyvinyl alcohol gels.
Unlike the systems used in the Kavesh et al. applications and Smith and Lemstra patents, all three versions of Zwick's Phase Separation process take up the fiber directly from the air gap, without a quench bath, such that the draw-down occurred over a relatively large length of cooling fiber.
U.S. Pat. Nos. 4,599,267 and 4,440,711 describe a process for preparing fibers composed of a linear ultra-high molecular weight polyvinyl alcohol.
Polyester and polyamide fibers and processes for forming such fibers are known. For example, the preparation and properties of nylon 6 and nylon 66 fibers are described in "Man Made Fibers, Science and Technology," Vol. 2. H. F. Mark et al., Eds., Interscience, N.Y., 1968. Polyester Fibers and Spinning Processes are described in Vol. 3 of the same work. In discussing spinneretes, it is said, "The capillary diameters usually range from 0.2 to 0.3 mm and their height ranges from 1 to 3 times the diameter." From a rheological point of view, the spinneretes must be properly considered as holes in a plate" p. 258, lines 1-4, "Man Made Fibers Science and Technology," Vol. 2, H. F. Mark et al., Eds , Interscience, N.Y., 1968.
Methods of preparing high tenacity, high modulus fibers have previously been described in U.S. Pat. Nos. 4,413,110, 4,440,711, 4,551,296 and 4,599,267. It was disclosed that, the length of the spinning aperature in the flow direction should normally be at least about 10 times the diameter of the aperature, or other similar major axis, preferably at least 15 times and more preferably at least 20 times the diameter, or other similar major axis. Such L/D (length/diameter) ratios of about 20/1 for the spinneret were within the bounds of prior art. See for example, "Man Made Fibers, Science and Technology Vol 1, p. 39, Interscience Publishers, N.Y., 1967.
Use of a die of 576:140 was investigated in connection with a process to produce a transparent polyethylene fiber. T.Y.T. Tam in a Ph.D. Thesis entitled "Continuous Extrusion and Orientation of Transparent Polyethylene Fiber", Ohio University, 1975 found that continuous extrusion was not possible with the high L/D die under the conditions of the investigation.